1. Field of the Invention
The present invention relates, in general, to a plate type steam reformer and, in particular, to a compact plate type steam reformer having a modularized structure, which can generate steam as well as heat a reforming reactor by use of heat from a burner, preheat high and low temperature water gas shift reactors by use of remaining waste heat after the reforming reactor is sufficiently heated, and properly control high and low temperature water gas shift reactors in temperature by positioning dampers in a flow path of exhaust gas to control a flow amount of the exhaust gas flowing in contact with exterior surfaces of reactors or by introducing cool air from outside to exterior surfaces of reactors.
2. Description of the Prior Art
Hydrogen has been generally used in the chemical engineering industry for processes such as desulfurization of crude oil and production of ammonia or chemical fertilizer, in the food processing industry for things such as production of low fat margarine, and in the metallurgical and iron industry for heat treating metal, and can also be applied to fuel for an automobile and a fuel cell as well as produce semiconductors, glass and optical fiber.
Recently, the demand has gradually increased for devices capable of continuously supplying hydrogen in the spot such as in the fuel cell or a hydrogen car. In practice, hydrogen is mostly produced by reforming natural gas or hydrocarbon, and also by reformation of naphtha, gasification of coal, electrolysis of water, and biomass.
Meanwhile, raw material may be reformed by various reforming processes such as steam reforming, oxygen reforming, and steam-oxygen mixed reforming processes, and in commercial use, hydrogen is mostly produced by the steam reforming process.
Used in the steam reforming process, a reformer generally comprises a steam generator, a desulfurizer, a reforming reactor, and high and low temperature water gas shift reactors. The reformer may further comprise a device for oxidizing carbon monoxide so as to effectively remove carbon monoxide with the purpose of using the reformer as a hydrogen source for the fuel cell, and a PSA (Pressure Swing Adsorption) device for providing high purity hydrogen by removing carbon dioxide and other impurities.
With reference to FIG. 1, the conventional reformer used in a large hydrogen plant, comprising a tube type reactor in a large furnace is illustrated, in which a reforming reaction is accomplished by 60 to 70% heat efficiency at a high temperature of 900 to 1000° C. Also, a reforming reactor, high and low temperature water gas shift reactors, a heat exchanger, and a steam generator are separately positioned outside of the reformer, and thus steam reforming process is large, complicated, and total heat efficiency is low due to heat loss from pipes.
Because a reforming reaction, and high and low temperature water gas shift reactions have different reactivities depending on temperatures of catalytic beds, respectively, each reactor has an optimum operation temperature. For example, the optimum operation temperature for the steam reforming reaction, which is a strong endothermic reaction, ranges from 700 to 900° C., and the optimum temperatures for the high and low temperature water gas shift reactions, which are weak exothermic reactions, range from 400 to 500° C. and 200 to 300° C.
Because the reforming reaction is an endothermic reaction, heat should be applied to the reforming reactor by a heat source such as a burner and electric heating element during the reforming reaction. In the case of using the burner as the heat source, waste heat from reforming reaction can be partially recovered through a heat exchanger but most of the heat cannot be utilized and is wasted.
High and low temperature water gas shift reactions, which are weak exothermic reactions, initially require a heat source such as the electric heating element in order to properly preheat the high and low temperature water gas shift reactors. However, in practice, preparative time for cooling is about 2 hours when the reactors are used according to the above prior art, and so the above prior art cannot be applied to the fuel cell or another hydrogen supply apparatus which requires a short preheating time.
Therefore, to avoid the above disadvantages, much effort has been made to produce a device comprising the reforming reactor and the high and low temperature water gas shift reactors, which is structurally simple and compact in size, as well as has a short preheating time and high energy efficiency.
For example, reference may be made to U.S. Pat. Nos. 6,203,587 and 5,733,347, which disclose a fuel gas steam reformer assemblage which is compact and lighter in weight than conventional steam reformer assemblages used in fuel cell power plants. The fuel gas steam reformer having a plate type structure is characterized in that any one side of the reformer is heated by a catalyst combustion type heater and the structure of the reformer is symmetric to be effectively heated by a reaction heat. In addition, the reformer cannot produce steam by itself because it does not have a device for removing carbon monoxide, and so the reformer needs some peripheral devices, thereby the fuel gas steam reformer has disadvantages in that it forms a bulky assembly in conjunction with peripheral devices.
In addition, U.S. Pat. Nos. 5,609,834, 5,180,561, and 5,015,444 introduce a plate type reformer assembly, in which a catalyst combustion burner is a heat source and heat is effectively applied from the heat source to a reforming catalyst bed. However, this assembly does not comprise a steam generator and water gas shift reactor, and so the assembly requires additional devices, therefore the assembly cannot be sufficiently compact.
Furthermore, Korean Pat. Laid-open Publication Nos. 1997-25688 and 1999-14655 disclose a reformer for generating hydrogen from natural gas. The former has disadvantages in that the reformer does not comprise a water gas shift reactor even though heat produced from catalytic combustion in a cylindrical reformer is used in a reforming reaction and steam generation, and thus the reformer cannot have a sufficiently compact structure. In addition, the later discloses a reformer, in which steam is generated, fuel is preheated by a single-type catalyst combustion burner, and exhaust gas heats a desulfurizer. However, because high and low temperature water gas shift reactors should be separately positioned outside of the reformer, an assembly needed to produce hydrogen becomes not compact but bulky.
As described above, when water gas shift reactors, a heat exchanger, and a steam generator are separately positioned from a reforming reactor, a reforming assembly which is compact in size and has a high energy efficiency, cannot be obtained. Therefore, a device integrating reactors is needed in order to reduce the size of the reforming assembly.
Korean Pat. Laid-open Publication No. 2000-22546 discloses a reforming device integrating reactors, in which cylindrical or quadrilateral reactors are concentrically positioned, an innermost space is for combustion, a reforming reactor is adjacent outside of the space for combustion, and a water gas shift reactor and oxidation reactor for removing carbon monoxide are positioned outside of the reforming reactor. According to this invention, a catalyst combustion burner or flame combustion burner is used as a heat source, and steam is produced by supplying water into a coil-shaped heat exchanger or a flow path positioned on an exterior wall of a burner in order to use combustion exhaust gas. Additionally, direction of flow of the exhaust gas is changed by opening and/or closing one or two covers positioned at a flow path of the exhaust gas, and thus a flow amount of the exhaust gas flowing into the water gas shift reactor and the oxidation reactor is controlled, thereby temperatures of reactors are controlled.
Accordingly, fuel is effectively used because heat occurring in combustion of fuel is applied to the steam generator for generating steam and the reforming reactor, and energy is effectively utilized because waste heat is applied to preheat water gas shift reactors.
However, even though inventors of this prior art assert that heat is effectively utilized because reactors are concentrically positioned, the reforming device has disadvantages in that the exhaust gas cannot naturally and sufficiently flow into a desired space because combustion exhaust gas flows vertically, and so temperatures of reactors are not effectively controlled and heat may accumulate in reactors. As for utilizing exhaust gas, opening and/or closing means, positioned at an exhaust port for controlling temperatures of the water gas shift reactor and the oxidation reactor, are complicated because the opening and/or closing means have a dual structure.
Other disadvantages of the above reforming device are that a temperature of a catalytic bed is difficult to uniformly maintain because a portion of the catalytic bed, to which fuel and air are applied, is highest in temperature and a temperature gradient is longitudinally formed in the catalytic bed owing to catalytic combustion when heat is applied through a catalytic combustion process, as well a burner should be used in order to provide additional heat to reactors because the catalyst combustion burner is restricted in volume and cannot provide sufficient heat to the reactors, and so the reforming device becomes complicated and bulky.
In the case of using the flame burner, a sufficient combustion space is needed because the cylindrical reforming device is vertically positioned, and so the device becomes larger.